1. Field of the Invention
The present invention relates to a method for forming a metallic wiring pattern, and particularly, to a method for forming a metallic pattern using a refractory metal such as tungsten (W) as a wiring material.
2. Description of the Related Art
In proportion to size reduction in LSIs (Large Scale Integration), tungsten wiring is increasingly employed as metallic wiring since tungsten wiring patterns have small electric resistance and superior thermostability. Tungsten films are, however, readily affected by light reflection from the film surface during exposure in a lithographic process, and therefore, resolution deteriorates. Due to this, fine patterning of tungsten films by a photoresist technique has been extremely difficult. As a remedy for this, a method has been investigated, in which an antireflection film is initially formed on a tungsten film, and a resist pattern is formed thereon. Typical examples of such antireflection films include inorganic types and organic types.
For example, TiN antireflection films are known as of inorganic type. The use of such a TiN antireflection film, however, cannot completely prevent light reflection due to unevenness of the tungsten film surface itself which generates film thickness variation in the direction perpendicular to incident light. Accordingly, in cases of tungsten films, complete antireflection can rarely be achieved by employing an antireflection film based on an inorganic material such as TiN.
On the other hand, application of an organic material to the surface of a tungsten film is known as a method using an organic antireflection film. In this method, an organic material containing a coloring matter which can absorb light having the exposure wavelengths is applied to the entire surface of a tungsten film beforehand. In this case, since the coloring matter is light absorbent in the exposure wavelength range, antireflection can be achieved when the film thickness is sufficient. Next, a typical conventional method for forming a tungsten wiring pattern using such an organic antireflection film will be illustrated with reference to FIG. 5.
As is shown in FIG. 5, a silicon oxide film 12 is initially formed on a substrate 10, and a tungsten film 16 is formed by CVD (Chemical Vapor Deposition) on the silicon oxide film 12 with a titanium-based intermediate film 14 intervening therebetween. Hereupon, this titanium-based intermediate film 14 intervening between the silicon oxide film 12 and the tungsten film 16 is disposed for improving adhesion between the silicon oxide film 12 and the tungsten film 16.
Subsequently, an organic antireflection film 18 is applied to the tungsten film 16. At this time, although the tungsten film 16 has surface unevenness, a smooth surface can be achieved by application of the organic antireflection film 18. Additionally, a resist 20 is formed on the organic antireflection film 18 having such a smoothed surface by lithography so as to be patterned in accordance with a wiring pattern. At this time, since the organic antireflection film 18 is formed on the tungsten film 16, reflection of exposure light due to surface unevenness of the tungsten film can be reduced, pattern resolution can be improved, and a resist 20 patterned in accordance with a predetermined fine wiring pattern can be formed.
Next, the organic antireflection film 18 and the tungsten film 16 are selectively dry-etched using the resist 20 patterned in accordance with the wiring pattern as a mask to form a predetermined fine tungsten wiring pattern.
Incidentally, in order to form a predetermined fine tungsten wiring pattern by the above-described conventional method for forming a tungsten wiring pattern using an organic antireflection film as shown in FIG. 5, the selective dry-etching of the organic antireflection film 18 and the tungsten film 16 should be carried out so as not to cause narrowing of the resist 20 used as a mask patterned in accordance with a predetermined wiring pattern.
Such selective etching is, however, accompanied by some difficulties. For example, when an oxygen-based etching gas is used to etch the organic antireflection film 18 and the tungsten film 16 using the resist 20 as a mask, the etching process should necessarily be performed under a low pressure with a high ionic energy level. In an etching process with a high ionic energy level, however, the underlying tungsten film 16 is sputtered at the time of etching off the organic antireflection film 18, the sputtered tungsten adheres to the side walls of the resist 20 to form tungsten adhering portions 22, and thus the resist pattern substantially becomes thicker.
In order to prevent such an event, the amount of tungsten sputtered from the tungsten film 16 should be decreased by reducing the ionic energy level. With a reduced ionic energy level, however, narrowing of the resist pattern by oxygen radicals can occur.
As an alternative method, suppression of over-etching can also suppress sputtering of the tungsten film 16 in the etching process with a high ionic energy level. This method is, however, defective in a case where the organic tungsten film 18 remains in depressions on the surface of the tungsten film 16 since such residual portions serve as masks to undesirably form remaining non-etched portions having the shape of a needle.